1. Field of the Invention
The present invention relates to a wireless communication system using Orthogonal Frequency Division Multiplexing (OFDM). More particularly, the present invention relates to a method and apparatus for adaptively allocating resources in a multi-user OFDM system.
2. Description of the Related Art
These days, many wireless communication technologies have been proposed as candidates to support high-speed mobile communication. Among them, Orthogonal Frequency Division Multiplexing (OFDM), which is a type of multicarrier modulation, has been recognized as the most promising next-generation wireless communication technology.
OFDM is a scheme of parallel-converting a serial input symbol stream and modulating the parallel-converted symbol streams with orthogonal subcarriers. The OFDM scheme may be applied to a variety of digital transmission technologies requiring high-speed data transmission, such as Wireless Internet, Digital Audio Broadcasting (DAB), Digital Multimedia Broadcasting (DMB), and Wireless Local Area Network (WLAN).
FIG. 1 is a diagram illustrating an OFDM transceiver chain according to the related art.
Referring to FIG. 1, in a transmitter chain 110 in a communication system using OFDM technology, control signals or data 111 are modulated into a series of modulation symbols by a modulator 112, and then converted from a serial signal into parallel signals by a Serial-to-Parallel (S/P) converter 113.
A subcarrier mapper 114 maps parallel signals output from the S/P converter 113 to subcarriers in a frequency domain. An Inverse Fast Fourier Transform (IFFT) unit 115 is used to transform the frequency-domain parallel signals into a plurality of time-domain OFDM symbols and provide the symbols to a Parallel-to-Serial (P/S) converter 116. A Cyclic Prefix (CP) or Zero Prefix (ZP) is added to each of OFDM symbols by a CP inserter 117 to eliminate or reduce the impact of multipath fading. The resulting signal is transmitted by a transmitting preprocessor 118 such as an antenna (not shown), or by a wire or cable.
In a receiver chain 120, if perfect time/frequency synchronization is acquired, a signal received by a receiving preprocessor 121 undergoes CP removal in a CP remover 122 and is provided to an S/P converter 123. A Fast Fourier Transform (FFT) unit 124 transforms the received signal from the time domain to the frequency domain, and the transformed signal is processed by a subcarrier demapper 125, a P/S converter 126 and a demodulator 127.
In the OFDM system, each OFDM symbol includes a plurality of subcarriers, and the subcarriers in the OFDM symbols carry modulation symbols.
FIG. 2 is a diagram illustrating an example of an OFDM transmission scheme using a first subcarrier, a second subcarrier, and a third subcarrier according to the related art. Since each OFDM symbol has finite duration in the time domain, subcarriers may overlap in the frequency domain.
As illustrated in FIG. 2, if perfect frequency synchronization is acquired in the transceiver, orthogonality may be maintained at sampling frequencies. If a frequency offset occurs due to imperfect frequency synchronization or high mobility, the orthogonality between subcarriers is destroyed at the sampling frequencies, causing Inter-Carrier Interference (ICI).
FIG. 3 is a diagram illustrating examples of OFDM symbols transmitted/received in a time domain according to the related art. Reference numeral 301 represents a transmission signal, and reference numeral 303 represents a received signal.
Because of the multipath fading, a CP part of the received signal 303 is often damaged by a previous OFDM symbol. However, if a CP is long enough, the received OFDM symbol with the CP must include only its signal convoluted by the multipath fading channel. Generally, the receiver performs FFT, for subsequent processing in the frequency domain. Compared with other transmission schemes, OFDM is advantageously robust against multipath fading. The multipath fading in the time domain is transformed into frequency-selective fading in the frequency domain. Due to the addition of the CP or ZP, Inter-Symbol Interference (ISI) between adjacent OFDM symbols is cancelled or significantly reduced. In addition, because each modulation symbol is transmitted in a narrow bandwidth, the modulation symbol experiences single-path fading. To solve the frequency-selective fading, a simple equalization scheme may be used.
In a multi-user OFDM system which allocates different subcarriers to user terminals, each of the user terminals suffers from independent fading, making it possible to obtain multi-user diversity effects. Therefore, compared with a scheme of allocating a bandwidth to a single user terminal, the multi-user OFDM system may efficiently use wireless resources.
Also, the multi-user OFDM system may maximize the total data rate of the system by being combined with schemes such as Adaptive Modulation and Coding (AMC), which employs different modulation levels and coding rates for subcarriers according to the channel states of user terminals, and Water Filling which is a transmitting power control scheme. In addition, for the multi-user OFDM system, various resource allocation methods have been proposed to maximize the total data rate of the system under several conditions such as transmit power and multipath fading. However, existing resource allocation methods still do not maximize the data rate of the communication system.
Accordingly, there is a need for a subcarrier and power allocation method for maximizing the total data rate of the system while satisfying an inter-user (or inter-user terminal) required transmission ratio in a multi-user OFDM system.